Homogeneous ethylene/.alpha.-olefin interpolymers are characterized by narrow molecular weight distributions and narrow short-chain branching distributions. Further, homogeneous ethylene interpolymers containing long-chain branches, known as "substantially linear" ethylene polymers, are disclosed and claimed in U.S. Pat. No. 5,272,236 and in U.S. Pat. No. 5,278,272, each of which is incorporated herein by reference.
The absence of low molecular weight, waxy components and the ability to evenly distribute comonomer has enabled the production of high quality elastomers, such as ethylene/propylene, ethylene/butene, and ethylene/octene elastomers, etc. However, as homogeneous linear and substantially linear ethylene polymers lack the highly linear fraction characteristic of heterogeneously branched polyethylene (and thus the high crystalline melting peak), homogeneous linear and substantially linear ethylene polymers tend to have a poorer high temperature resistance, especially when the polymer density is less than 0.920 g/cm.sup.3, than heterogeneously branched polymers of the same density. For instance, homogeneous linear and substantially linear elastomers may lose their strength at 60.degree. C. or less. This has been attributed to the fact that such low density polymers have a molecular structure which is characterized by the presence of fringed micelles, and typically lacks higher melting point lamellar structures. While the differential is less pronounced, even higher density homogeneous linear and substantially linear ethylene polymers which have lamellae structures, generally melt at lower temperatures than their heterogeneously branched counterparts. Regardless of polymerization catalyst, polyethylenes face a practical use limitation above their crystalline melting point, which does not exceed approximately 140.degree. C.
Through blending high crystallinity grades of polyethylene with low crystallinity elastomeric grades, it is possible to raise the use temperature of the elastomeric grades. However, greater improvements in high temperature resistance are desired. Further, generally speaking, however, as the amount of the higher density fraction increases, the high temperature resistance increases, while the modulus increases (and thus, the elastomeric properties, in the case of blends with homogeneous linear or substantially linear ethylene polymers having a density less than 0.900 g/cm.sup.3, undesirably decreases). In the case of blends with homogeneous linear and substantially linear ethylene polymers having a density greater than 0.900 g/cm.sup.3, as the amount of the higher density fraction increases, the high temperature resistance increases, while the tear resistance and impact resistance undesirably decrease.
U.S. Pat. No. 5,530,072 discloses polymers exhibiting long chain branching formed by self-grafting a linear polyethylene using a free radical initiator. While such self-grafting serves to increase the molecular weight of the polyethylene and to improve the melt strength, it does not affect the crystallinity of the polyethylene, and thus does not affect the high temperature resistance of the polyethylene.
U.S. Pat. No. 5,346,963 discloses graft modified substantially linear ethylene polymers, which are optionally blended with thermoplastic polymers, such as high density polyethylene, linear low density polyethylene, and low density polyethylene.
Industry would find advantage in an elastomer which has enhanced high temperature performance without sacrifice of modulus and/or tear resistance and impact resistance. Such enhanced high temperature performance may show advantage, for instance, in shoe soles which better withstand the heat of a clothes dryer. In another embodiment, such enhanced high temperature performance may show advantage, for instance, in pressure sensitive adhesives which exhibit reduced creep resistance.